Cell cultures will be established (Methods of Paterson, M.D. et al, Nature 260:444-447,1976) and examined for mycoplasma contamination by assaying exogenous 3-H-uracil/3-H-uridine incorporation into RNA (method of Schneider, E.L. etal, J. Expl. Cell Res. 84:311-318, 1974). If cells fail to grow or show contamination, Principal Investigator will immediately alert Project Officer for repeat specimen. An initial assay of in vitro radiosensitivity will be performed. Colony formation after irradiation will be measured and results compared with colony formation of control and ataxia-telangiectasia fribroblasts (method of Lehman, A.R. et al, Nature, 258:427-429, 1975). On all cell lines with abnormal radiosensitivity and other lines selected by mutual agreement, further assessment of DNA repair will be performed. Monolayer cultures in late logarithmic growth phase will be irradiated anoxically and monitored for ability to execute 2 enzymatic DNA repair processess.